This grant application requests funding for the purchase of a shared Multiphoton Confocal Microscope System for the Intellectual and Developmental Disabilities Research Center (IDDRC) Imaging Core at Children's Hospital, Boston. This microscope offers state-of-the-art technology that will allow many investigators to visualize fluorescent structures deep into living and fixed tissues at unprecedented spatial resolution and with high sensitivity. These features of the multiphoton confocal system will benefit the growing number of NIH- funded research studies at Children's Hospital and the Harvard Medical School Area that require the imaging of cellular processes deep in live tissue. Research programs that require visualization of fine processes of neuronal networks in situ will also profit strongly from access to this instrument. The modern multiphoton confocal system will be incorporated into the Cellular Imaging Core Facility located in the Department of Neurology and F.M. Kirby Neurobiology Center at Children's Hospital. This facility has been operational for over 34 years and serves more than 74 research laboratories at Children's Hospital and throughout Harvard Medical School. Scientists and clinicians in this research community will be able to use this microscope to study a wide variety of problems in neuroscience and other fields of biology, including how the mechanisms of synaptic plasticity, neural-glia interactions, axon guidance defects, pain, stem cell proliferation, sensory system signaling, and organ system development relate to human disease.